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Mating for life? Not for the birds and bees. Of 5,000 species of mammals only 3 to 5%, including humans, are monogamous

Last week, researchers in Argentina revealed a moving story - Magellanic penguins that they had been tracking for 30 years are monogamous despite the most incredible odds. Inhabitants of southern Argentina, Chile and the Falkland Islands, male-female pairs of these penguins stick it out with each other for most of their 25 year life, even though pop spends six months away in the sea, travelling up to 16, 000 kms away from mom who is looking after the eggs. Ultimately he returns, finding her by specific calls.

The story made waves because humans like to hear of monogamy - and the implied undying love, fidelity and steadfastness - in other species. The reality of sexual and familial bonds in animals and birds is, however, quite the opposite. There are about 5, 000 species of mammals. Of these only 3 to 5 percent are known to practise lifelong monogamy. Included in this select group are beavers, otters, wolves, some bats and foxes and a few hoofed animals, apart from humans. Scientists say that this was the rough proportion amongst humans too in primitive times when they lived in groups, surviving through hunting and gathering. In all probability, monogamy arose when agriculture developed because it was then that the need to preserve property through lineage arose. You couldn't identify your offspring unless the pair bond was fixed.

Monogamy means something else for animals and birds. Among humans, it implies both social and sexual exclusiveness. But this isn't always the case with other animals.

For example, it is estimated that 90 percent of all birds are socially monogamous, that is, pairs live together, raising their young jointly. But it has now been discovered that many birds may have sex with other partners frequently. In a famous experiment female blackbirds paired with sterilised males were still laying eggs that hatched. Clearly, they were making out on the side, but stuck to hubby for raising the brood. Even gibbons and swans, thought to be loyal mates have been found to cheat and even divorce on occasion.

Scientists have intensively studied some species that are rigidly monogamous. One is the black vulture found in the Americas only. Their code is so strong that when "extra-pair copulation takes place nearby, vultures will attack the philanderer", according to a report.

But the advantages of monogamy for the Black Vulture are also clear: parents staying together means both incubate the eggs for 24-hours each, and both feed the fledglings for eight months. In fact this is the main evolutionary reason in favour of monogamy, scientists think - that the offspring will be better fed, better protected and hence have a better chance of survival.

This could out-weigh the evolutionary advantages of polygamy - having several sexual partners - such as spreading your genes far and wide so that chances of dead end from a weak partner are minimised.

Research done on another virtuous and loyal animal, the mouse-like prairie vole, found in central US, shows that a male vole will "prefer to mate exclusively with the first female he loses his virginity to". Such is the fire of loyalty in him that a mated male vole will attack other females, forget about flings or even mild flirtation. Scientists have found that certain neurotransmitters in the rodents' brains, including dopamine, are connected to this behaviour.

One reason why monogamy may be the preferred option in humans is that the human newborn takes a much longer time to fully develop than all other animals. The human newborn is much more defenseless and dependant on external support. Two parents in a steady relationship would be necessary for better development and happiness.

Staying faithful can be a struggle for most animals. For one, males are hardwired to spread their genes and females try to seek the best dad for their young. Also, monogamy is costly because it requires an individual to place their entire reproductive investment on the fitness of their mate. Putting all their eggs in one basket means there's a lot of pressure on each animal to pick the perfect mate, which, as humans knows, can be tricky.